Sudoku difficulty levels
What Easy, Medium, Hard, Unfair and Extreme mean, and what the number next to them is.
Hodoku Revisited rates every puzzle the way the original HoDoKu does: it solves the puzzle with the easiest technique available at each step and records which techniques it had to use. The level reflects the hardest techniques the puzzle needs, so an Easy puzzle never requires anything beyond singles, while an Extreme one cannot be finished without a forcing technique.
Easy
Only singles are needed: a cell that is the last empty one in its unit, a cell with one candidate, or a digit with only one place in a unit.
Techniques at this level: Full House, Naked Single, Hidden Single.
Medium
Singles are not enough. You need to remove candidates with locked candidates and with naked and hidden pairs and triples.
Techniques at this level: Locked Candidates, Naked Pair, Triple and Quad, Hidden Pair, Triple and Quad.
Hard
Patterns that span the grid: fish, single-digit patterns, wings, coloring, uniqueness arguments and short chains.
Techniques at this level: X-Wing, Swordfish and Jellyfish, Skyscraper, 2-String Kite, Empty Rectangle, XY-Wing, XYZ-Wing, W-Wing, Simple Colors, Uniqueness techniques, Turbot Fish, Multi Colors, Finned and Sashimi Fish, Hidden and Avoidable Rectangles, BUG+1.
Unfair
Long chains and loops, almost locked sets, and fish with fins. Some of these are laborious to find by eye.
Techniques at this level: XY-Chain and Remote Pairs, ALS-XZ, Nice Loops and AICs, X-Chain, Franken and Mutant Fish, Sue de Coq, ALS-XY-Wing, ALS-Chain, Death Blossom.
Extreme
Techniques that try an assumption and follow its consequences: forcing chains and nets, Kraken fish and templates. Only a computer finds these comfortably.
Techniques at this level: Kraken Fish, Templates (for computers), Forcing Chains and Nets.
The number next to the level
Each technique carries a score, and the puzzle's score adds up the scores of all the steps on its solution path. Two puzzles of the same level can therefore differ: a long Hard puzzle can have a higher score than a short one. In the app the rating appears at the top as, for example, Hard · 612. A higher number means more or harder work.
Choosing a level
If you are new, start at Easy and move up when you finish a puzzle without hints. If you want practice on a particular technique, use the Learning and Practicing modes, which build puzzles around the techniques you pick. The hint setting Hints use caps how hard a hint may be, which is handy when you only want to be reminded of the simpler tricks.